As a gas sensor for detecting hydrogen gas or others, for example, there is a semiconductor gas sensor having a MOS (Metal-Oxide-Semiconductor) structure using a catalytic metal as a gate.
For example, Japanese Patent Application Laid-Open Publication No. 2009-300297 (Patent Document 1) discloses a gate structure which includes a Ti-modified film obtained by mixing oxygen-doped titanium containing oxygen and a titanium fine crystal, and a platinum film made by a plurality of crystal grains formed on the Ti-modified film, and in which oxygen and titanium exist in a crystal boundary region among the plurality of crystal grains.
In addition, Japanese Patent Application Laid-Open Publication No. 2012-073154 (Patent Document 2) discloses a structure which includes a gate insulating film provided on a substrate and a gate electrode provided on the gate insulating film, and in which the gate electrode has a metal-oxide mixed film obtained by mixing an oxygen-doped amorphous metal containing oxygen and an oxide crystal of the metal and a platinum film provided on the metal-oxide mixed film.
In addition, International Patent Publication WO 11/055605 (Patent Document 3) discloses a MISFET hydrogen gas sensor of low power consumption which can operate over one year by a low voltage power source. The document describes a technique in which a sensor FET is formed in a MEMS region obtained by hollowing a Si substrate of an SOI substrate, and in which a heater wiring is arranged so as to fold between a Pt—Ti—O gate and source electrode and between the Pt—Ti—O gate and drain electrode of the sensor FET.
In addition, Japanese Patent Application Laid-Open Publication No. 2010-008371 (Patent Document 4) discloses a flammable gas sensor which includes a sensor chip, a stem on which the sensor chip is mounted, and a metal cap having an upper portion and a side portion and having the bottom of the side portion welded to the peripheral of the stem, and in which the sensor chip is surrounded by the stem and the metal cap.